[flow] report doc count when uploading file

This fixes
#478
There was no `markStarted` and so 'remaining' was always at 0, so the
frontend thought the job was done, even though it was getting
notification about 'succeeded' in that job increasing
4 jobs for 477-dev-flow-zip-file-upload in 37 minutes and 28 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Cabal
passed #18395
cabal

00:15:21

 
  Stack
passed #18396
stack

00:02:46

 
  Bench
manual #18397
allowed to fail manual
bench
 
  Test
passed #18398
test

00:19:21